fix: Console.log Statements Left in Production Code

[Aditya8369]

What I changed

• Added Winston dependency in package.json and updated package-lock.json.
• Created shared logger at logger.js with JSON logs, timestamps, stack support, and environment-based log level.
• Replaced console logging with logger calls (info/warn/error + context metadata) in:
  • auth.controller.js
  • group.controller.js
  • message.controller.js
  • privacy.controller.js
  • socket.js
  • db.js
  • email.js
  • auth.middleware.js
  • index.js
  • user.seed.js

Verification performed

• Ran install: npm install winston in backend.
• Searched for remaining console usage in backend source: no console.* matches found in backend/src.
• Checked diagnostics on backend/src: no errors found.

closes #73

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